Cryptography for Beginners Guide

@adam_englander
Cryptography for Beginners
Adam Englander
Software Architect, iovation

@adam_englander
I am a Virtual Crime Fighter

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I am a lover of PHP

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I Am Not…
• … a security researcher
• … a cryptographer
• … a mathematician

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What To Expect
• Gain a working understanding of common
terms used in cryptography.
• Understand the key drivers for choosing
cryptography methodologies, algorithms and
strengths.
• Know which PHP modules to use.

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What Not to Expect
• Deep dive into algorithms
• A lot of comparison between algorithms and
implementations
• Become a cryptography expert

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What Is Cryptography?
Cryptography…is the practice and study of
techniques for secure communication in
the presence of third parties called
adversaries.
Wikipedia

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My Deﬁnition of Cryptography
Cryptography obscures data in such a way
that it is difficult and costly to duplicate
or reverse.

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How do you make it difﬁcult?

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–Oxford Dictionary
Entropy:
Lack of order or predictability; gradual
decline into disorder.”

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The greater the entropy; the
greater the difﬁculty.

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Achieving Maximum Entropy
• Use Cryptographically Secure Pseudo-Random Number
Generators (CSPRNG).
• Salts add global randomness to hashing.
• Feedback loops add local randomness to block ciphers.
• Initialization Vectors add global randomness to block
ciphers.
• Some ciphers introduce randomness with padding.

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Local vs Global Entropy
Local entropy is
entropy with a
singular pice of data
within a larger
system.
Global entropy is
entropy of the same
or similar data across
the entirety of a larger
system.

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How Random Is Random?
• It turns out it can be quite random as long as you
use the correct tools.
• Since PHP7, CSPRNG extension provides platform
independent cryptographically secure pseudo-
random data.
• Until you move to PHP7, paragonie/random_compat
package will give you the same functionality.

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How do you make it expensive?

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Algorithms that are difﬁcult to
crack with brute force

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What Contributes to Cost?
Secrets
Computation
Entropy

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Computational Cost
• Complexity of algorithm increases cost.
• Key length increases cost.
• Some algorithms speciﬁcally target memory
and thread utilization to increase cost.
• Feedback loops increase cost.

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Cryptography is based on
ciphers

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Encryption Signatures
Key Derivation Hashing

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Asymmetric Encryption
• Based on very large
prime numbers
• Computationally
expensive
• Padding for entropy
• Cannot encrypt data
larger the the key

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Asymmetric Key Cryptography
• RSA and DSA are available in PHP. Use RSA.
• Uses very large prime integers
• Very computationally expensive
• Uses key pairs to protect secret

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Super Duper Secret
• Private key can do encrypt, decrypt, sign, and
verify signature
• Public key does not have enough data to
decrypt or sign. Can only encrypt and verify
signature

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Key Size and Hash Algorithm
• Current minimum recommend key size is 2048
• SHA1 is considered safe but SHA-256 is better

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Data Limitations
• RSA can only encrypt or sign data up to the
length of the key size
• Signatures use hashing
• Crypto often mixed with symmetric key
cryptography

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Padding
• Padding is how RSA creates additional
entropy
• Use Optimal Asymmetric Encryption Padding
(OAEP)
• Do not use PKCS1-V1_5 as it is no longer
considered cryptographically secure

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Symmetric Encryption
• Uses small shared key
• Has modes for
encrypting data larger
than the key
• Fast encryption
• Uses initialization vector
and key for entropy

Block Cipher Modes
DO NOT USE Electronic Cookbook (ECB)!!!

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Block cipher modes determine how the
blocks of clear text are translated into
cipher text.
What are Block Cipher Modes?

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Cipher Block Chaining (CBC)
• Entire message is required for decryption
• Full cipher text block is used as the seed for
the next block

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Cipher Block Chaining (CBC)

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Galois Counter Mode (GCM)
• Counter based cipher stream
• Entire message is required for decryption
• Encrypts plain text and generates an
authentication code similar to an HMAC
simultaneously that is returned with the IV in
the cipher text

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Galois Counter Mode (GCM)

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Hashes by themselves aren’t very useful!

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Digital Signature
• Used to verify integrity of data
• Used mostly for data transfer
• Can be used for verifying data at rest
• Can not be reversed but can be reproduced
for veriﬁcation

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Digital Signatures (HMAC)
• Hash-based Message Authentication Code
(HMAC)
• Hashing combined with key
• SHA-256 or better is preferred to ensure
uniqueness

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Key Derivation

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Key Derivation
• Uses salt for entropy
• Iterates to increase cost
• Can create cost via threads and memory
• Bigger is better!

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Which KDF Should I Use?
• argon2i is the new hotness
• scrypt is preferred
• bcrypt is acceptable
• PBKDF2 can be used in a pinch

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How Can I Use KDFs?
• PHP has the best package for managing that
PERIOD! Use the password extension. Just
use it!
• For *cough* pre-5.5.0, you can use ircmaxell/
password-compat
• Provides tools for hash upgrades. AWESOME!

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And now…an example

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Hi Bob, I’m Alice.
Can you hear me?
Bob Alice
Client starts
TCP session by
sending a
synchronized
packet to the
server

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Bob Alice
Yes Alice, I can
hear you. Can you
hear me?
Server responds by
sending SYNchronize-
ACKnowledgement
packet to the client

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Yes Bob, I can hear
you.
Bob Alice
Client completes
TCP session the
initialization by
sending
ACKnowledge
packet to the
server

@adam_englander
This is
conversation
12345. I know
German and
Spanish.
Bob Alice
Client informs the
server the session
requires TLS and
sends the TLS version
number, cipher
settings, public key,
and session-speciﬁc
data.

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Bob Alice
Let’s use German.
Here’s my ID.
Server chooses
the cipher
settings and
sends
Certiﬁcate.

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Bob Alice
The
signature on his
ID matches. He’s
Bob.
Client verifies
the server’s
certificate by
verifying the
signature
against the
known
certificate
authority.

@adam_englander
Ich denke an
eine Farbe, die Sie
mit gelb zu
machen.
Bob Alice
Client and
server begin a
conversation
that is now
encrypted using
asymmetric
encryption.

@adam_englander
The rest of the conversation would
be in German. But, we’ll show it in
English.

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I have a color
that makes orange
with yellow with
yellow.
Bob Alice
Session symmetric key
negotiation begins with
client generating a
secret random value
and sending a shared
value and a value
derived from the two.
Alice’s secret is red, the
shared value is yellow,
and the derived value is
orange.

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Bob Alice
I have a color
than makes green
with yellow.
The server generates its
own secret random
value and sends a
value derived from it
and the shared value.
Bobs secret is blue, the
shared value is yellow,
and the derived value is
green.

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Bob Alice
Our shared color
is purple.
Both determine a
new shared
value based on
combining the
two secret
values.
Alice’s secret is
red. Bob’s secret
is blue. Their
combined
secrets are
purple.

@adam_englander
ログインページを
教えてください
Bob Alice
Client sends HTTP
request the server
for the login page
using symmetric
encryption with the
newly negotiated
key.

@adam_englander
Bob Alice
ここで、ログイン
ページです
Server sends an HTTP
response with the
login page HTML
using symmetric
encryption with the
newly negotiated key
to the client.

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Recommendations

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Disclaimers
• Although every app is different, commonalities
exist across most applications
• I am only recommending what I know and have
vetted directly or indirectly via my work
experience
• If you think you are different, ask yourself if the
advantages outweigh the risks

@adam_englander
Types
• Use RSA asymmetric key cryptography when
transferring data
• Mix with AES and random keys/IVs per transfer
• Use CSPRNG extension/package for keys, salts
and initialization vectors
• Use password extension/package for passwords

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Strength
• Use the strongest cryptography you can afford
• AES: aes-256-cbc / sha256 minimum
• RSA: 2048+ PKCS1_OAEP / RSA-SHA256
• Hash until it hurts!

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Packages/Libraries
• Use OpenSSL for encryption and digital signatures
• For extreme compatibility, use phpseclib/phpseclib
• Use CSPRNG extension/package for keys, salts
and initialization vectors
• Use password extension/package for passwords

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MCrypt
DO NOT USE THIS!
IT IS DEPRECATED!
IF YOU ARE USING THIS. PLAN YOUR
MIGRATION AWAY NOW!

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Password Hashing
• password_hash to hash
• password_verify to check password against the
hashed value
• password_needs_rehash to ssh if the hash
needs updated

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Hash
• hash_hmac for hashing - sha256 for algorithm

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OpenSSL
• AES-CBC or AES-GCM
• RSA for asymmetric encryption
• PBKDF2 for key derivation - not so great

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libsodium
• AES-GCM or ChaCha20-Poly1305 for symmetric
encryption including auth tag
• XSalsa20-Poly1305 for asymmetric encryption
• Ed25519 for asymmetric digital signatures
• Blake2b for hashing
• Argon2 and Scrypt KDFs for password hashing

@adam_englander
Resources
• https://secure.php.net/manual/en/book.openssl.php
• https://secure.php.net/manual/en/book.csprng.php
• https://secure.php.net/manual/en/
book.password.php
• https://packagist.org/packages/phpseclib/phpseclib
• https://en.wikipedia.org/wiki/Cryptography

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